Plasticizer system for propellant compositions

ABSTRACT

This invention discloses the synthesis of a polytetramethyleneglycol pivalate polyether and its use as a plasticizer for propellant compositions.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

BACKGROUND OF THE INVENTION

This invention relates to solid propellant compositions and to animproved internal plasticizer system for use therewith. In a moreparticular aspect, this invention concerns itself with a novel family ofbranch-chained, saturated polyethers of particular molecular structureand to their use as an internal plasticizer for isocyanate-curedpropellant compositions.

The increased interest and utilization of solid propellant compositionshas spawned a considerable research effort in an attempt to improvetheir ballistic and physical properties. Generally, solid propellantsare composed of one or more organic or inorganic oxidizers dispersed ina resinous binder matrix which may also function as a fuel. Typicaloxidizers are ammonium perchlorate or HMX (cyclotetramethylenetetranitramine), both of which are well known in the art. Variousresinous components, such as hydracarbons, polyesters, polyurethanes andother like materials may serve as a binder/fuel matrix. A supplementalfuel component, such as finely powdered aluminum, may be used also.Other additive components, such as anti-oxidants, burning ratemodifiers, wetting agents, anti-foaming agents and plasticizers may beadded to the propellant composition, if desired. Dibutylphthalate,dioctyl adipate, or triacetin are often employed as inert plasticizersin combination with the resinous binder material.

Plasticizers are used in rocket propellants for a number of reasons.These include processing assistance by incorporation of fluid materialsin the propellant mix, the improvement of low temperature flexibilityand the improvement of mechanical properties and ballisticcharacteristics. In using solid propellants, however, a problem existsin the use of plasticizers because of the tendency of plasticizermolecules to migrate or evaporate. Either of these processes results inchemical changes in the composition which are harmful to the propellantand to other inert parts of the rocket motor. On long term storage,changes in mechanical properties of the propellant often occur,particularly near the liner-propellant bond. Other negative effectswhich are associated with the use of plasticizers, includecrystallization at low temperatures, a tendency to soften the propellantexcessively at high temperatures, and the fact that the plasticizer maymigrate into the liner form the uncured propellant much more rapidlythan it does from a cured propellant system. Consequently, a continuingresearch effort has been conducted in an attempt to solve the migrationproblem associated with propellant plasticizers, and to provide aplasticizer that does not migrate rapidly even in an uncured propellant.

One research effort involved the use of a material known as ZL-496. Thismaterial, ZL-496, is a polybutadiene with a molecular weight ofapproximately 3,000. The use of ZL-496, as a plasticizer, wascontemplated because of its molecular size. It is quite large and it wasbelieved that its polymer chain entanglement would prevent migration.Unfortunately, however, it was found that even plasticizer molecules aslarge as ZL-496 have a tendency to migrate out of the propellantcomposition which will ultimately shorten the shelf life of a rocketmotor.

In furthering the above research effort, however, it was unexpectdlydiscovered that a branch-chained, saturated polyether of particularmolecular structure could be utilized as a plasticizer in thefabrication of solid propellants. This novel plasticizer replaces theconventional plasticizers generally utilized in composite propellants,especially those having a binder base which utilizes an isocyanate curereaction for its curing system. The resulting propellant exhibits aminimum amount of plasticizer migration during normal storage andexhibits a very good shelf-life.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided a family of novelcompound which act effectively as plasticizers for propellantcompositions without the attendant problems of plasticizer migrationwhich often arise during propellant storage. The plasticizer of thisinvention are polyethers having a particular moldecular structure asillustrated by the following structure formula. ##STR1## wherein n is aninteger of from 6 to 12.

These plasticizers are used in the propellant in a binder/plasticizerratio ranging from about 75 to 95 parts by weight binder to 5 to 25parts by weight of plasticizer.

Accordingly, the primary object of this invention is to provide a novelfamily of polyether plasticizers having a particular molecularstructure.

Another object of this invention is to provide a novel solid propellantcomposition that produces only minimum amounts of plasticizer migrationduring its shelf-life.

Still another object of this invention is to provide a novel family ofpivalate ether plasticizers that find particular utility as internalplasticizers for solid propellant compositions.

The above and still other objects and advantages of the presentinvention will become more readily apparent upon consideration of thefollowing detailed description thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With the above-mentioned and other objects in mind, the presentinvention contemplates the synthesis of a novel family of pivalate etherplasticizers and their utilization as internal plasticizers in aconventional composite solid propellant.

The novel family of plasticizers contemplated by the present inventionare branch-chained, saturated, polytetramethyleneglycol pivalate ethershaving the following structural formula. ##STR2## wherein n is aninteger of from 6 to 12.

It is believed that the particular molecular structure illustrated inFormula (I) above provides a particular advantage for the use of thesepolyethers as internal plasticizers. The plasticizer links to the bindernetwork of the propellant and, therefor, is unable to migrate orevaporate away from the propellant during storage. This provides thepropellant with an excellent shelf-life.

The synthesis of the plasticizer of this invention is accomplished byeffecting a reaction between an equimolar mixture of pivaloyl chlorideand Teracol. This reaction is illustrated by the following reactionscheme. ##STR3## wherein n is an integer of from 6 to 12.

Example 1, which follows, discloses the experimental details of thereaction and method of synthesis illustrated by equation (II).

EXAMPLE (I) Polytetramethyleneglycol pivalate

360 grams of (0.554 moles) of Teracol (a hydroxy-terminated polyetherwith a molecular weight of approximately 650 g/mole) was added to 1200ml of dry ether and 141 ml of triethyl amine in a 3-liter flask andcooled in an ice bath. 66.7 grams (0.554 moles) of pivaloyl chloride(MW=120.45) was dissolved in 100 ml of dry ether and added drop-wise tothe Teracol solution over a two hour period. The reaction mixture wasstirred overnight at room temperature. Ether was added as necessary tomaintain volume. The reaction mixture was then cooled in an ice bath andsuction filtered to remove Et₃ N·HCL. The ether layer was washed twicewith 50 ml portions of 10% H₂ SO₄, twice with 50 ml portions ofsaturated Na₂ CO₃ solution, and twice with 50 ml portions of distilledwater. The pH of the washed product was obtained to assure itsneutrality. The ether layer was separated and dried overnight over 250 gof Na₂ SO₄. The ether was removed from the product by rotaryevaporation.

An illustration showing the use of the novel internal plasticizer ofthis invention in a solid propellant is shown in Table I as follows.Although ammonium perhlorate and isocyanate cured polybutadiane binderare preferred, other conventional oxidizing and resinous binders may beutilized, if desired, as well as other conventional propellantcomponents.

Solid propellant compositions are well known and since the basicpreparation and constituent ingredients of the propellant compositionsof this invention are not significantly altered or critical to theexecution of the invention, with the exception of the internalplasticizer component, a detailed explanation of the propellant'spreparation is not deemed necessary. The plasticizers of this inventionare pre-reacted with the isocyanate curing agent, and then incorporatedinto the propellant mix in a conventional manner at any state prior tocure. Generally, all of the ingredients are homogenously mixed in aconventional blender. After mixing, the uncured propellant mix is thenplaced in an oven and cured at a temperature and for a period of timesufficient to produce a firm, rubbery, solid propellant. The resultingpropellant differs from a conventional propellant composition only inthe essential replacement of a conventional plasticizer with the novelinternal plasticizer of this invention.

TABLE I

68.0 parts by weight of ammonium perchlorate

7.45 parts by weight of hydroxy-terminated polybutadiene (R-45M)

22.0 parts by weight of aluminum powder fuel

0.25 parts by weight of a curing agent, isophorone diisocyanate (IPDI)

2.0 parts by weight of the internal plasticizer of this invention whichwas first prereacted with IPDI

0.30 parts by weight of a bonding agent, HX-752

0.075 parts by weight of a delayed quick-cure catalyst (equal parts MgO,Triphenyl Bismuth, and Maleic Anhydride)

The above ingredients were thoroughly mixed in a conventional mixeruntil a homogenous mixture had been obtained. The uncured propellant wasthen placed in an oven at 170° F. for 7 days. After that period of time;a firm, rubbery, solid propellant was obtained.

Table II, which follows, illustrates a conventional propellantcomposition using dioctyl adipate (DOA) as an internal plasticizer witha propellant of the type exemplified in Table I.

                  TABLE II                                                        ______________________________________                                        FORMULATION OF DOA ANALOG                                                     PLASTICIZED PROPELLANT                                                        INGREDIENT     PARTS BY WEIGHT OF MIX                                         ______________________________________                                        DOA            2.0                                                            MAGNESIUM OXIDE                                                                              0.025                                                          ALUMINUM       22.0                                                           AP 200         51.0                                                           AP 6900        17.0                                                           MALEIC ANHYDRIDE                                                                             0.025                                                          TRIPHENYL BISMUTH                                                                            0.025                                                          R45M BINDER    8.0                                                            ______________________________________                                    

Tables III and IV provide gel fractions and viscosities of thepropellants of Tables I and II while Table V sets forth the physicalproperties of the two propellant mixtures, for purposes of comparison.It can be seen that the plasticizers of this invention provide thepropellant with physical characteristics as good or better than dioctyladipate while simultaneously providing a solution to the problem ofplasticizer migration.

                  TABLE III                                                       ______________________________________                                        GEL FRACTIONS OF ANALOG                                                       PLASTICIZED PROPELLANTS                                                                             GEL        AMBIENT                                      Propellant MIX NO.    FRACTION   STRESS                                       ______________________________________                                        No Plasticizer                                                                           8823T      0.413      103                                          Table II   8774T      0.461      162                                          Table I    219        0.349      100                                                     227        0.839      300                                          ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        VISCOSITIES OF PLASTICIZERS IN R-45M                                                            Plasticizer                                                 Temp °F.                                                                      R-45M      of Table II                                                                             Plasticizer of Table I                            ______________________________________                                        128° F.                                                                       293.sup.2  .sup. 146.sup.1                                                                         282                                                74° F.                                                                       1281       448       729                                                28° F.                                                                       6520       1808      OFF SCALE                                         ______________________________________                                         .sup.1 20% PLASTICIZER USED IN R45M.                                          .sup.2 ALL VISCOSITIES GIVEN IN CPS.                                     

                                      TABLE V                                     __________________________________________________________________________    PHYSICAL PROPERTIES OF PLASTICIZED PROPELLANTS                                __________________________________________________________________________             -65° F. TEST TEMPERATURE                                                             STRAIN                                                 Propellant                                                                             STRESS        MAX*/FAIL**                                                                           MODULUS                                        __________________________________________________________________________    of Table II                                                                             730           9/17   20,400                                         of Table I                                                                             1170          3/3     60,000                                         __________________________________________________________________________    AMBIENT TEST TEMPERATURE 165° , TEST TEMPERATURE                            STRESS                                                                             MAX*/FAIL**                                                                           MODULUS                                                                              STRESS                                                                             MAX*/FAIL**                                                                           MODULUS                                 __________________________________________________________________________    Table II                                                                           162  26/28   1310   133  25/26   2280                                    Table I                                                                            100  21/30   1260                                                        __________________________________________________________________________     *STRAIN AT MAX STRESS                                                         **RUPTURE STRAIN                                                              Table VI discloses additional physical properties of the propellant           composition of Tables I and II.                                          

                                      TABLE VI                                    __________________________________________________________________________    PHYSICAL PROPERTIES OF ANALOG PLASTICIZED PROPELLANTS                         __________________________________________________________________________                   Binder to                                                                           -65° F. Test Temperature                                     Cure                                                                              Plasticizer                                                                             Strain                                               Propellant                                                                          Mix No.                                                                            Ratio                                                                             Ratio Stress                                                                            Max*/Fail**                                                                          Modulus                                       __________________________________________________________________________    of Table II                                                                         118  0.77                                                                              80:20                                                                117  0.80                                                                              80:20                                                                119  0.83                                                                              80:20                                                                8774T                                                                              0.77                                                                              80:20  730                                                                              8.5/17 20,400                                        of Table I                                                                          216  0.85                                                                              90:10 1065                                                                              5.0/5.0                                                                              44,800                                              217  0.95                                                                              85:15                                                          __________________________________________________________________________    Ambient Test Temperature                                                                       165° F., Test Temperature                                  Strain          Strain      Shore                                        Stress                                                                             Max*/Fail**                                                                          Modulus                                                                            Stress                                                                            Max*/Fail**                                                                          Modulus                                                                            A                                            __________________________________________________________________________    Table II                                                                      134  22/24  1060                 67                                           134  15/16  1330                 62                                           212   9.2/10.2                                                                            4776                 78                                           162  26/28  1310 133 25/26  1180 72                                           Table I                                                                       129  19/43  1560  81 27/29   530 69                                            85  18/36  1100                 53                                           __________________________________________________________________________

While the present invention has been described by reference toparticular embodiments, it should be understood by those skilled in theart that all the modifications that are embodied within the scope of theappended claims are intended to be included herein.

What is claimed is:
 1. A solid propellant composition comprising a curedhomogenous mixture of:A. a solid, particulate oxidizer component; B. asynthetic resinous, polyisocyanate cured, polybutadiene bindercomponent; and C. a polytetramethyleneglycol pivalate ester plasticizerhaving the following structual formula: ##STR4## wherein n is an integerof from 6 to 12 and said plasticizer is present in a binder/plasticizerratio ranging from about 75 to 95 parts by weight of binder to 5 to 25parts by weight of plasticizer.